The present invention relates generally to the field of powder metallurgy, and more particularly to articles formed from compositions of matter that include a tungsten-containing powder and at least one binder.
Conventionally, many articles have been produced from lead because of lead""s relatively high density (11.3 g/cc) and relatively inexpensive cost. Examples of such articles include firearms projectiles, radiation shields and various weights. More recently, lead substitutes have been sought because of the toxicity of lead. For example, in 1996 the Environmental Protection Agency banned the use of lead shotgun shot for hunting waterfowl. Various lead substitutes have been used, including steel, bismuth and tungsten, with each offering various advantages and disadvantages as compared to lead.
One solution to the toxicity of lead is to form articles from a lead-substitute, such as tungsten or a tungsten alloy. Although such tungsten-containing articles may possess desirable properties, including relatively high densities, many tungsten mixtures and alloys are also hard and abrasive, which makes machining and shaping of such articles difficult. In order to effectively shape such articles, tools that are also extremely hard are required. For example, tools may need to be made or edged with tungsten carbide, such as tungsten carbide in a cobalt matrix. Unfortunately, although tungsten carbide is very hard, it is also relatively brittle. As a result, besides being more expensive, such tools typically have a shorter life. For example, attempts to shape or work tungsten-containing articles using thin-edged tungsten carbide tools or punches have frequently resulted in damage to the tools themselves, at a substantial cost.
The present invention is directed to manufacturing processes for articles that are formed from powders containing tungsten and at least one binder. The manufacturing process includes compacting the mixture of powders under a first pressure to yield a desired intermediate structure, then reshaping the structure under a second pressure that is lower than the first pressure to yield the desired article. Appropriately durable tools may be used for the high-pressure compaction step, while more precise tools may be used for the lower-pressure reforming step.
In some embodiments, the manufactured article contains at least one metallic binder. In some embodiments, the article contains at least one non-metallic binder, such as a polymeric binder. In some embodiments, the article contains both a metallic binder and a non-metallic binder. In some embodiments the article is a lead substitute. In some embodiments the article is a firearms projectile, such as a bullet or shot, which may be ferromagnetic or non-ferromagnetic, which may be frangible or infrangible, and which may be jacketed or unjacketed.